Katie I beg to differ on a few points. I've been a parent at Sackville since 2005 so have a pretty good knowledge of the place. The Independent Schools Inspectorate (Ofsted equiv for indies) rates it as excellent. ISI don't have an outstanding category. Excellent is the top grade. The latest report is less than a year old.
"Interaction with local indies" - ds3 (y10) has been in football, rugby, basketball, volleyball and cricket teams and played more indies than state schools in matches so not sure where you got your info from.
"Not a school many able dc attend" Your info is several years out of date. One of ds1's peers has gone to Oxford and several over the past few years are at RG unis or in the USA. Its a mixed ability school so has people across the ability range. Since Cognita took over about 7-8 years ago and the new head arrived, results have gone up and up and I think you will find that we were the highest performing local non - selective either last year or the year before I lose track.
"several children with spld being encouraged to leave the school" Absolute . I assume you are referring to one student who 2 years ago left - if you know this student you will know why the school (and most others) was wholly inappropriate. ds3 is on the sn register
The school won't suit everyone - no school will. Every place has its own personality. It is small place and girls are in a 30/70 minority mainly because it was originally a boys school and there are a lot of girls' indies around. The only indie taking boys only is Tonbridge whereas there are 4 girls only indies in the vicinity. Its a no fripperies place - they are not concerned with having the latest astroturf and drama studios, they spend their money on getting the best teachers they can.
The school is very different from when we started in 2005 so when Katie says unless much has changed in the last few years - the answer is yes - much has!
OP, PM if you have any specific questions.